Method of making highly reactive ion-leachable glass

ABSTRACT

A method is provided for producing a highly reactive, ion-leachable glass for cement compositions. The method contemplates blending together a mixture of ion-leachable, inorganic compositions, forming the blended mixtures into shaped charges, heating the shaped charges to form a homogeneous melt, and finally blowing, in a gaseous mixture, to form partially solidified thin glass fibers. The method uniquely produces glass having a degree of crystallinity of less than 1 percent by weight in crystalline form.

This is a division of application Ser. No. 203,761, filed Nov. 7, 1980,now U.S. Pat. No. 4,401,773, granted Aug. 30, 1983.

BACKGROUND OF THE INVENTION

This invention relates to ion-leachable inorganic compounds in the formof glasses and suitable for use as components of cements.

Ion-leachable inorganic compounds such as the oxides of aluminum, zinc,mangesium and calcium have been intermixed with other components such assilica and formed into glasses which, when combined with such hydrogendonating compounds such as acids, will set up into a cementitious mass.The mechanism for the reaction has been described by Alan D. Wilson etal. (J. Dent. Res. 58(3), 1065-1071, March 1979) and may be representedby the generic equation.

    ______________________________________                                        MO       +     H.sub.2 A   =   MA      +   H.sub.2 O                          ion-leachable  Proton donating Salt                                           inorganic      compound        Hydrogel                                       compound                                                                      ______________________________________                                    

Cements utilizing this mechanism have generally taken the form of glasspowders incorporating the ion-leachable inorganic. These are reactedwith liquid acid solutions such as aqueous carboxylic acid solutions toform a salt hydrogel structure which sets up into a hard mass. Suchcement forming compositions have been suggested for use in applicationssuch as dental cements and for orthopedic casts and splints. Forexample, a fluoroaluminosilicate glass powder has been suggested for useas the ion-leachable component for a dental cement in British Pat. No.1,316,129. More recently, a similar composition has been suggested foruse in orthopedic surgery in U.S. Pat. No. 4,143,018.

In using such compositions for orthopedic purposes, for example, certaincriteria must be met. The composition when rendered reactive, must becapable of providing sufficient "working time", i.e., sufficient timefrom the start of mixing the reactants to allow the doctor time to applyand mold the cast into shape before the material reaches a stage whereit is no longer malleable. Generally such times should be at most about4 minutes and preferably from 1 to 2 minutes.

At the end of the working time period, it is most desirable that thecast set to a rock-like state as quickly as possible. While mostcements, even after attaining a rock-like appearance, do not reach theirultimate strength for long periods of time, the material should reachsufficient compressive strength to allow a patient to leave the doctor'soffice, i.e., sufficiently hard enough to preclude deformation underexpected stresses. This period is referred to as the "setting time" andshould be about 6 to about 15 minutes after the cast is applied.

Within the frame work of providing practical working and setting times,perhaps the most important criterion to the user of such cements ispredictability. When dealing with a patient, the orthopedist must beable to rely on the manufacturer's directions for predicting how muchtime he has to form a cast and when the patient will be free to leavethe office.

From the foregoing it becomes apparent that a composition must beprovided wherein the rate at which the salt hydrogel reaction proceedscorresponds to the constraints of working and setting times and ishighly predictable and reproducible. As with most reactions, the rate isgenerally a factor of temperature and the availability of rate-limitingcomponents which in this case is the availability of the leached metalions from the powdered glass component. In practice, temperature is notgenerally a controlled factor in that the practitioner is accustomed tousing aqueous mixture of components at essentially room temperature.

It thus becomes apparent that the manufacturer must supply a compositionin which the rate of leaching of metallic ions is the controlling factorin meeting the various criteria of a satisfactory product. It is in thisconnection that the use of ion-leachable glass powders have heretoforebeen found wanting. Powders produced have varied greatly with respect totheir physical and chemical homogeneity which in turn has producederratic and unacceptable variations in working and setting times. Inparticular, it has been discovered that conventional glass frit makingprocedures have produced glass powders in which the components areseparated into phases of different compositions, in which themicro-structure of the glass has been uneven, i.e., regions of amorphousmaterial surrounding ordered regions of high crystallization, and ingeneral, high degree of variance in amorphous to crystalline structure.While an exact correlation between rate of ion-leachability andmicro-structure of glass powders is not known, reported studies haveshown such a relationship between structure and rate, such beingreported by T. I. Barry, et al. in J. Dent. Res. 58(3): 1072-1079, March1979. It has been discovered from this and other work that the rate ofion-leaching is substantially decreased when the degree of crystallinityof a glass is increased.

Accordingly, there is a need for providing glass powders in which thedegree of crystallinity can be reproducibly controlled and predicted.

SUMMARY OF THE INVENTION

In accordance with this invention glass is produced which is highlyreactive and for which the rate of hydrogel forming reaction is bothreproducible and controllable. Specifically, a shaped charge of amixture of ion-leachable inorganic compounds is heated to form asubstantially homogenous melt. The melt is next blown in a gaseousmedium to form at least a partially solidified stream of thin glassfibers. The fibers then solidify and may be ground into a powder havinga degree of crystallinity of less than about 1% by weight in thecrystalline form.

It has been discovered that perhaps the prime factor in the variation inreactivity of various glasses is related to a corresponding variation inthe degree of crystallinity. Accordingly, if this variable isessentially eliminated, the rate of reaction may be accuratelycontrolled and predicted by simply adjusting the mixture of theinorganic components used in making the glass. It has been found thatthe most expedient way of eliminating the variation in crystallinity isto provide a glass to the reaction that is essentially all amorphous,i.e., has an extremely low degree of crystallinity and preferably adegree of crystallinity wherein less than about 1% by weight of theglass is in the crystalline state. In accordance with the teachingsherein such a glass can be provided by performing the specific steps setout above as contrasted with conventional glass frit making processeswhich produce erratic and undesirable degrees of crystallinity.

Preferably, the glass fibers are formed by a compressed air stream atessentially room temperature and have a diameter which may vary from0.004 to 0.06 mm. The homogeneity of the melted glass should bemaintained as closely as is possible. To aid in homogeneity, it isimportant that rather than charging the mixture of inorganic componentsdirectly to the melting stage, the components first be blended and thenextruded into strands or other shaped charge for the melter. Additivessuch as extrusion lubricants and binders may be intermixed with theinorganic components and vaporized away in a calcining step prior tocharging the melter. Generally, it is sufficient to allow the glassfibers to solidify in air and the resulting product will have the lowdegree of crystallinity herein prescribed. It should be noted, however,that in some cases, certain glass compositions tend to crystallize veryrapidly. Said in other words, crystals tend to form at temperatures verynear the melt point. Such glass compositions are, for example, glassescontaining MgO as a starting material. In these cases, it is preferablethat solidification of the fibers be accelerated to a rate higher thanmere air cooling. This can be accomplished by having the glass fibersfall directly into a water quench tank and then be dried prior togrinding.

The solidified glass in any event is ground to a powder having aparticle size distribution such as to pass through a 325 mesh screen,i.e., less than 0.044 mm, with 50% being smaller than 0.015 mm indiameter. In this manner, a reactive glass powder is provided having nomore than about 1% by weight in the crystalline state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a specific embodiment of theprocess for making glass in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to FIG. 1, illustrated there is a schematic flow diagramof a process for producing the glass of this invention. In accordancewith this invention, a shaped charge of mixture of inorganic componentsis homogeneously prepared, melted in a furnace, formed into solidifiedglass fibers, and ground into powder.

Preparing Homogeneous Mixture

The feed materials, illustrated in FIG. 1, as streams 1, 2, and 3comprise one or more ion-leachable inorganic compounds such as theoxides of alkali, alkaline earth, aluminum, and zinc metals along withsilica. The silica preferred is in the form of finely ground quartz.Aluminum, in the form of alumina, and zinc compounds are preferablyintroduced as powdered oxides. Alkalies and alkaline earths may beintroduced in the form of carbonates. These particulate materials arethoroughly blended in a mixing station until uniform. The mixing stationpreferred is a Muller-type mixer.

To aid in producing a uniform feed to the furnace, it is important thatthe mixed inorganic compounds be fabricated into shaped forms such as,for example, extruded strands. Accordingly, additives may be introducedat the mixing station for facilitating such extrusions. In theembodiment illustrated in FIG. 1, stream 4 represents the addition of alubricant such as the aqueous wax dispersion sold by Mobil Corporationunder the tradename "Mobil Cr-C". Stream 5 represents the addition of abinder to the mixture to give the extruded product structural integrity.Such a binder may be, for example, a hydrophilic material such asammonium alginate which is available under the tradename "Superloid"from the Kelco Company of Clark, N.J., U.S.A.

Stream 6, the mixture of ion-leachable compounds, silica, binders andlubricant is next passed to an extruder where it is extruded, at ambienttemperature through a 10 mm die the produce stream 7, strands havingdiameters of from about 2 mm to about 15 mm.

The extruded strands, stream 7, are next charged to a sagger, i.e., afire-clay box of the kind in which pottery is packed for firing toovens. The sagger is heated at approximately 900° C. for about 4 hoursduring which time the binder and lubricant are vaporized away as well asthe carbonates and water of hydration present in the inorganic feedmaterial. These latter materials pass out of the sagger as carbondioxide and water vapor in stream 8. This process, essentiallycalcination, reduces the extruded strands to a mass of friable materialhaving substantially a homogeneous distribution of the various chemicalentities.

Preparing the Melt

The calcined material, stream 9, next passes to a furnace where it ismelted into molten glass. Satisfactory results have been obtained byusing electric melting although other means could be employed. Anelectric furnace has been successfully utilized which consists of adouble hulled steel shell having cooling water, streams 10 and 11,circulated between the shells. It is preferred that no refractory liningbe used as such use will increase the chances for contamination andimpair the homogeneity of the melt. Fortunately, it has been discoveredthat the batch fed to the furnace can act as its own refractory lining.Power is transmitted to the batch material in the furnace by means oftwo graphite electrodes.

The preferred operation sequence is to bring the electrodes together atthe surface of the unmelted batch and then separate the electrodes tocreate an arc. The arc melts a pool of batch material adjacent to thearc. When enough molten material has been formed, the electrodes arelowered into this pool and the heating of the molten material isdeveloped by resistance heating, without any open arc, and with heatbeing carried throughout the mass by conduction.

Solidifying the Melt

When sufficient melt has accumulated in the furnace, one or more streamsof melt are poured from the furnace through an orifice to form meltstreams having diameters from about 12 to about 25 mm, such a streambeing represented in FIG. 1 by stream 15. The stream 15 is met with oneor more streams of compressed air at essentially ambient temperatures,e.g., streams 12 and 13, and blown thereby into fine glass fibers.Preferably, the fine glass fibers have an average diameter of from about0.008 to about 0.01 mm. with a maximum diameter of about 0.04 mm. Thecompressed air stream is at conditions of temperature and pressure andpositioned relative to stream 15 in any manner such as to best producethe glass fibers described above. These conditions are functions of suchfactors as the particular furnace being used, the physical properties ofthe melt (with viscosity being the prime factor) and other factorspeculiar to a specific embodiment of this invention. Typically, thecompressed air stream will be at a pressure of from about 40-80 poundsper square inch, gauge, and will be placed about 25 mm from the stream15.

If the composition of the glass mix is such as will crystallize rapidly,the air blown fibers, stream 14, are quenched immediately in a waterquench, preferably by allowing them to simply fall into the quench tank.Generally, this step may be omitted provided that crystallization is nottoo rapid to produce the low degree of crystallinity prescribed herein.The quenched fibers, stream 16, are then passed to a drying and millingstation where they are ball milled into a powder. Preferably aluminumoxide pebbles are employed in the mill and the fibers are ground to apowder which will pass through a 325 mesh screen, i.e., the largestdimension of the powder particles is less than 0.044 mm. A typicalparticle size distribution obtained by the process of this inventionresults in about 70% of the powder being less than 0.03 mm and 50% beingless than 0.012 mm.

The resulting glass powder has been found to be highly reactive andalmost totally amorphous. As a result, its use as a cement is totallypredictable and working and setting times may be controlled.

To more specifically illustrate the advantages of this invention thefollowing examples are given:

EXAMPLE 1

A series of glass samples are prepared by combining the followingingredients in the following proportions in a Multon-type mixer:

    ______________________________________                                        Component        Moles   Weight (gms)                                         ______________________________________                                        Silica           64      3845                                                 Aluminum         38.4    3915                                                 Calcium Carbonate                                                                              38.4    3843                                                 ______________________________________                                    

The mix is then transferred to an 8-inch diameter electric furnace ofthe type described above. An arc is struck at the surface of the batchbetween the one inch graphite electrode using 75 volts and a seriesresistor in the circuit. The arc starts melting the batch immediately incontact with it and when a sufficient pool is formed, the electrodes aremoved down into the pool so that further melting is by conduction in themelt.

When sufficient melt has accumulated, the melt is poured as a streamthrough a 200 mm diameter hole in the furnace wall. The melt streammeets with an air jet at a nozzle pressure of from 40 to 80 pounds perinch gauge, said nozzle being held about 25 mm from the melt stream andat an angle of about 30 degrees to the melt stream to form the glassfibers. The air stream also serves to direct the glass fibers into a 55gallon drum approximately half filled with water.

The fibers are collected from the drum, dried, and then placed in a ballmill containing aluminum oxide pebbles and milled overnight. Theresulting powder is screened dry through a 325 mesh screen. Four batchesof powder are produced in the above manner.

Each of the batches are tested for crystallinity by immersing to powderin an oil of known refractive index and observing the material through apetrographic microscope using a 45 power objective. As viewed betweencrossed Nicols, the crystal phase will appear light against a darkbackground. The percent of crystallinity for the four batches are foundto be 5, 40, 15 and 25%, respectively.

Cementitious masses are made from each of the glass batches by mixingthe glass powders together with the following ingredients:

1 gram glass powder

0.12 grams polyacrylic acid, molecular weight equals 154,000

0.05 grams d,l-tartaric acid (as an accelerator)

0.25 ml. water

Set times are determined to be the time in which the cementitious masshardens to the degree that the surface cannot be marred by gauging withone's fingernail. The set time for the four batches were 9, 32, 16, and25 minutes respectively.

As can be seen from the above results, both the degree of crystallinityand the set times varied widely and unpredictably.

EXAMPLE 2

The same ingredients, in the same proportions as are set out in Example1 above are combined in the Mueller type mixer with the exception that1728 grams of Mobil Cr-C lubricant and 2320 gram of Superloid binder areadded. The blend is then extruded at ambient temperature through a 10 mmdie and the extruded strands are placed in a sagger and heated in anoven at 800°-1100° C. for four hours. The contents of the sagger arethen transferred to the electric furnace described in Example 1 above,and treated in essentially the same manner.

Microscopic examinations show the glass powder to be essentially crystalfree with a maximum degree of crystallinity of less than 1% by weight.The fingernail test described above is performed on cementitious massesmade from batches of the glass powder and results in a substantiallyuniform set time of about 9 minutes.

What is claimed is:
 1. A method for producing highly reactiveion-leachable glass in cementitious compositions containing protondonating compounds comprising:blending together a mixture ofion-leachable inorganic compounds; forming said blended mixture intoshaped charges; heating said shaped charges to form a homogeneous melt;blowing said melt, in a gaseous medium to form at least partiallysolidified thin glass fibers having a degree of crystallinity of lessthan 1% by weight in the crystalline form and grinding the fibers. 2.The process of claim 1 wherein said gaseous medium is an air stream. 3.The process of claim 1 wherein said glass fibers have a diameter of from0.004 to 0.06 mm.
 4. The process of claim 1 wherein said shaped chargesare formed by extruding said ion-leachable inorganic components intostrands.
 5. The process of claim 4 wherein said ion-leachable inorganiccomponents are first blended together with extrusion lubricants prior tobeing formed into shaped charges.
 6. The process of claim 4 wherein saidion-leachable inorganic components are first blended together withbinders prior to being formed into shaped charges.
 7. The process ofclaim 1 wherein said shaped charges are first calcined prior to beingmelted.
 8. The process of claim 1 wherein said glass fibers are quenchedby falling directly into a quench tank.
 9. The process of claim 1wherein said glass is ground to a powder having a particle sizedistribution such as to pass through a 325 mesh screen with 70%, byweight being smaller than 0.03 mm in diameter.